The present invention relates to a phase change memory device and a method for manufacturing the same, and more particularly to a phase change memory device capable of reducing the size of a unit cell as well as raising the current flow in a transistor, and a method for manufacturing the same.
Memory devices may be categorized into a Random Access Memory (RAM) or a Read Only Memory (ROM). RAM devices are volatile memory devices (such as Dynamic RAM or DRAM, and Static RAM or SRAM), which will lose input information when power is off. The ROM devices are non-volatile memory devices (such as flash memory and Electrically Erasable Programmable ROM or EEPROM), which preserves the stored state of input information even when power is off.
Although DRAMS are generally considered very good memory devices in many ways, there are known difficulties for high integration as each DRAM requires a high charge storage capacity leading to increased electrode surface area. High integration is also considered to be difficult for flash memories as each flash memory having a stacked structure of two gates requires an operational voltage higher than its power source voltage and thus requires a separate booster circuit in order to establish the voltage necessary for write and erase operations.
Thus, studies were made to develop a new type of highly integratable non-volatile memory device having a simple and not unduly complicated structure. A phase change memory (in particular, a phase change RAM) is one such non-volatile memory device being researched.
A phase change memory device is a memory device in which the current flow between upper and lower electrodes causes the phase change layer interposed between the electrodes to undergo a phase change between a crystalline phase and an amorphous phase. The resistance difference of the phase change of the phase change layer is then used to discern the types of information stored in the memory cell.
More specifically, the phase change memory device uses a Chalcogenide layer, a compound layer of Germanium (Ge), Stibium (Sb), and Tellurium (Te), as the phase change layer. Joule heat generated through the application of a current causes the Chalcogenide layer to undergo a phase change between a crystalline phase and an amorphous phase. Here, because the phase change layer has a higher resistance when in the amorphous phase as compared to the crystalline phase, the phase change memory uses a read mode to distinguish whether the information stored in the phase change memory cell corresponds to logic “1” or logic “0” by detecting the current flowing through the phase change layer.
FIG. 1A is a graph illustrating a phase change of a phase change layer in a conventional phase change memory device.
As illustrated in the drawing, the phase change layer is changed to the amorphous phase by heating it at a temperature higher than the melting temperature (Tm) for a short time (first operation period: T1) and then cooling it at a rapid speed (see curve ‘A’). To the contrary, the phase change layer is changed to the crystalline phase by heating it at a temperature lower than the melting temperature (Tm) and higher than crystallization temperature (Tc) for a time (second operation period: T2) longer than the first operation period (T1) and then cooling it (see curve ‘B’).
Thus, in writing current necessary for the phase change of the phase change layer, it can be appreciated that a high current and a short pulse are required for the amorphous phase, and a low current and a long pulse are required for the crystalline phase.
FIG. 1B is a circuit diagram illustrating the conventional phase change memory device. As illustrated in the drawing, the phase change memory device includes a variable resistor C connected to a bit line BL and a word line WL connected between the variable resistor C and a ground voltage. The variable resistor C includes a lower electrode, a phase change layer, and an upper electrode.
It is necessary for such a phase change memory device to be primarily manufactured in a low cost, and have a unit cell of high integration or high density for realizing a high capacity.
In order to raise the integration or density of the unit cell, it is necessary to concentrate the current to a portion where the phase change layer and the lower electrode are in contact with each other to raise the current density. However, because a high current is required for the phase change of the phase change layer, a channel width of a transistor is necessarily increased, and thus a size of the unit cell should also be increased.
In addition, in the conventional phase change memory device, when forming a ground line using a damascene process, an etching damage to a lower portion layer is generated in a ground line area formed elongated like a bar when an etching process, which results in characteristic degradation of the finally obtained phase change memory device.